Motor-powered portable grinding machine

ABSTRACT

A motor-driven manual grinder ( 10 ), in particular an eccentric-plate grinder, has a work spindle ( 13 ) supported in the tool housing ( 11 ) and also has a grinding plate unit ( 15 ) that is driven and rotatable by the work spindle to execute an eccentric motion. The manual grinder ( 10 ) further has an annular first rolling face ( 28 ), extending together with the grinding plate unit ( 15 ) all the way around the eccentric axis ( 19 ), and an annular second rolling face ( 30 ), associated with the first, whose center axis extends coaxially to the central axis ( 17 ) of the work spindle ( 13 ), and on which the first rolling face ( 28 ) can roll. The second rolling face ( 30 ) is supported rotatably about its center axis. It is assigned a braking device ( 35 ), by means of which a rotation of the second rolling face ( 30 ) about its center axis can be selectively prevented for the forced-drive mode or enabled for the free-wheeling mode.

PRIOR ART

[0001] The invention is based on a motor-driven manual grinder, inparticular an eccentric-plate grinder, as generically defined by thepreamble to claim 1.

[0002] A manual grinder of this type is known (European PatentDisclosure EP 0 245 850 A2), in which the first rolling face is embodiedas an outer annular gear, which revolves with the driven grinding plateabout the axis thereof, and in which the second rolling face is formedof an inner annular gear on an external crown gear, which is retained ina manner fixed against relative rotation with respect to the toolhousing and is adjustable axially, by means of an external actuatingdevice, between an inoperative position and a functional position thatcooperates with the first rolling face. In the inoperative position, thesecond rolling face is out of engagement with the first rolling face. Inthe functional position, conversely, the second rolling face extends inapproximately the same diametrically opposed plane as the first rollingface, so that in the revolution of the grinding plate the first rollingface can roll along the second rolling face, and thus a motion thatrotates the grinding plate about its eccentric axis is superimposed onthe eccentric motion of the grinding plate. As a result, when the secondrolling face is in the functional position, enhanced abrasion of theworkpiece is attainable. A disadvantage of this manual grinder is thatthe repositioning between idling and the forced-drive mode is notpossible during operation of the manual grinder. In the axial relativemotion, damage can occur to the teeth of both rolling faces. Anotherdisadvantage, because of this arrangement of rolling faces, is arelatively great structural height of the grinder.

ADVANTAGES OF THE INVENTION

[0003] The motor-driven manual grinder of the invention, in particularan eccentric-plate grinder, having the characteristics of claim 1 hasthe advantage over the prior art that a switchover between thefree-wheeling mode and the forced-entrainment mode is possible duringoperation of the manual grinder without the risk of damage, and thestructural height is reduced at little expense for gearing.

[0004] By the provisions recited in the other claims, advantageousrefinements of and improvements to the motor-driven manual grinderdefined by claim 1 are possible.

[0005] In an advantageous embodiment, the braking device has aneccentric lever, actuatable by hand, for instance, that actuates thebrake member and that has an eccentric element acting on the brakemember.

[0006] The eccentric lever can be supported pivotably in the toolhousing and as its eccentric element can have an eccentric peg thatengages an opening, for instance a loop, on the end of the brake member,especially a brake band, which loops around the outer circumferentialface of the external crown gear.

[0007] In another advantageous embodiment, the eccentric lever has ahandle, located outside the tool housing, for pivoting actuation.Advantageously, the eccentric lever can be designed such that tensingthe brake band is done by means of pivoting about a circumferentialangle of about 180°. Thus the handle of the eccentric lever can easilybe pivoted between two positions on traversing a circumferential angleof 180°, and thus the repositioning between the forced-drive mode andthe free-wheeling mode can be done even during operation of the manualgrinder.

[0008] In another advantageous embodiment of the invention, the outerannular gear is a part, in particular an integral part, of the grindingplate unit, for instance an integral component of the grinding plateitself, onto which the outer annular gear is injection-molded. This isespecially simple and economical and furthermore contributes to reducingthe structural height.

[0009] In still another advantageous embodiment, the inner annular gearof the external crown gear has a higher number of teeth than the outerannular gear. The difference in the number of teeth can for instance be2. As a result, in the establishment of the forced-drive mode, thegrinding plate can be driven at a thus-specified rpm. For instance, ifthe number of oscillations is 10,000, then for a ratio of the number ofteeth of 50:48, the resultant rotary speed of a grinding plate in theforced-drive mode is 417 rpm.

[0010] In another advantageous embodiment, the grinding plate unit has asleeve, coupled and in particular connected in a manner fixed againstrelative rotation to the work spindle and having an eccentric peg, forinstance on its end, as well as a grinding plate retained on the peg bymeans of a bearing and connected detachably, for instance by means of ascrew, to the eccentric peg.

[0011] It can also be advantageous if the outer annular gear has abearing ring, axially spaced apart from the inner annular gear along thecenter axis, and is rotatably supported with the bearing ring by meansof a bearing relative to the sleeve that is coaxial to the work spindle.The bearing can for instance be pressed onto the sleeve, and theexternal crown gear is pressed with its bearing ring onto the outer ringof the bearing.

[0012] In yet another advantageous embodiment, a fan wheel of aninternal dust extractor is secured to the sleeve. Alternatively, the fanwheel can also be seated directly on the work spindle in a manner fixedagainst relative rotation and can have a sleeve that is eccentric to thespindle axis, with a cylindrical sleeve for terminal retention of thegrinding plate being rotatably supported in the eccentric sleeve bymeans of a bearing.

[0013] In another advantageous embodiment, the sleeve with the eccentricpeg on its end is formed of a sintered part and is thus designedespecially economically. It can also be advantageous if the externalcrown gear is formed of a lightweight metal or zinc die-cast part, whichonce again makes for an economical design.

[0014] In another advantageous embodiment, the grinding plate, with theouter annular gear integral with it and forming the first rolling face,is formed of a one-piece injection-molded part, making for still furthercost reduction and simplification.

[0015] Yet another advantageous embodiment provides that the brakemember can be locked relative to the tool housing in a first position,in which it is in positive engagement with the second rolling face, andin a second position, in which it is not in any engagement with thesecond rolling face. It is especially advantageously possible as aresult that the braking device is switchable between the first andsecond position and vice versa in all operating states, and inparticular during idling, at a stop, and under load.

[0016] It is especially advantageous if the brake member is a band, inparticular a toothed belt, with a plurality of teeth which are able toenter into engagement with a crown gear of the second rolling face.Compared to purely frictional engagement, in this case no slip occursbetween the braking device and the second rolling face. This preventswear of the two parts meshing with one another and suppresses heatproduction.

[0017] It is especially advantageous if the brake member is embodied asan elastic band, in particular as a toothed belt. By means of such anelastic intermediate coupling, the switchover from the free-wheelingmode to the forced- entrainment mode can be made easily, with littletolerance.

[0018] Preferably, the brake member, in particular the elastic element,is connected to the tool housing at a fixation point such that it isrotatable about a fixed rotary axis. As a result, upon motion of theelastic element between the first and second positions, it isunnecessary to kink the elastic band, which means less wear.

[0019] It is advantageous if the braking device has a detent lever,which is connected to the tool housing via a spring element, especiallyif the spring element seeks to press the detent lever into a position inwhich the elastic element assumes its first position. Such a design ismechanically easy to achieve and is nevertheless sufficiently stable, sothat incorrect operation will not occur.

[0020] It is also advantageous if the brake member is of spring-elasticmaterial and has a first recessed region with a set of teeth, which canbe brought into engagement with the outer circumferential face, and asecond recessed region, and the brake member is held by prestressing inits first position and can be brought into its second position by anactuating device. It is possible as a result to use a simple part, suchas a stamped part, as the brake member. This provides an economicalembodiment. Moreover, by means of such a design, simple actuation ispossible with an only slight actuating force. This embodimentsimultaneously functions as an overload protection for the forcedoperating mode and as a cushion against rotational impact. It requireslittle space; in addition, it becomes possible to compensate for errorsin pitch of the gear part, for instance caused by gear wear.

[0021] It is also advantageous if the detent lever is displaceable bymeans of an eccentric bolt, which is supported rotatably on the toolhousing and is operable by means of the actuating device protruding fromthe tool housing. Thus the user can very easily switch over between thetwo positions of the elastic element without having to turn off thedevice, regardless of the operating state it is in at that particulartime, that is both in the free-wheeling (fine grinding) mode and theforced-entrainment (coarse grinding) mode.

[0022] It is also advantageous if the fixation point and the eccentricbolt are essentially diametrically opposite one another relative to thecentral axis, and the angle between the fixation point and the teeth ofthe elastic element, which in the first position are in engagement withthe second rolling face, is greater than 90°. This prevents overlookingunder load.

[0023] It is furthermore advantageous if the fixation point and theeccentric bolt are essentially diametrically opposite one anotherrelative to the central axis, and the cooperating faces of the brakemember and of the second rolling face, under load, reinforce theretention force, similarly to a servo effect. Thus at little effort, amajor retaining force is generated, which increases the reliability ofthe apparatus.

[0024] It is also advantageous if the braking device is switchablebetween the first and second position and vice versa in all operatingstates, and in particular during idling, at a stop, and under load. As aresult, there is no need first to switch from one operating mode toanother to enable switching between the first and second positions. Thissaves time and makes for greater ease of use for the user.

[0025] Further advantages and details of the invention are the subjectof the dependent claims.

DRAWINGS

[0026] The invention is described in further detail below in terms oftwo exemplary embodiments shown in the drawings. Shown are:

[0027]FIG. 1, a schematic section through a first exemplary embodimentof a manual grinder;

[0028]FIG. 2, a schematic plan view of parts of a braking device of themanual grinder in the plane II-II in FIG. 1;

[0029]FIG. 3, a schematic fragmentary section taken along the lineIII-III in FIG. 1;

[0030]FIG. 4, a schematic view from below of parts of the manual grinderin the direction of the arrow IV in FIG. 1, without a grinding plateunit;

[0031]FIG. 5, a schematic section through a second exemplary embodimentof a manual grinder;

[0032]FIG. 6, a schematic section taken along the line A-A in FIG. 5, inwhich the braking device is not in engagement with the second rollingface;

[0033]FIG. 7, a schematic section as in FIG. 6, with the braking devicein engagement with the second rolling face; and

[0034]FIG. 8, a schematic fragmentary view of a third exemplaryembodiment of a braking device.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0035] In FIG. 1, a first exemplary embodiment of a motor-driven manualgrinder 10 is shown schematically; it is embodied here as aneccentric-plate grinder. The manual grinder 10 has a tool housing,identified overall by reference numeral 11, which includes an electricdrive motor 12 with a work spindle 13 that is supported in the toolhousing 11 by means of a bearing 14, for instance in the form of a ballbearing. A grinding plate unit identified overall by reference numeral15 is connected to the work spindle 13 in a manner fixed againstrelative rotation; it is driven by the work spindle 13 to execute aneccentric motion and is rotatable. The grinding plate unit 15 has asleeve 16, which is connected to the work spindle 13 axially and in amanner fixed against relative rotation. The sleeve 16 extends coaxiallyto the central axis 17 of the work spindle 13 and on its lower end, forinstance, in FIG. 1 it has an eccentric peg 18, whose eccentric axis 19extends with an eccentricity e to the central axis 17 and parallel toit. By means of a bearing 20, such as a ball bearing, a grinding plate21 is rotatably supported on the eccentric peg 18. The grinding plate 21is connected axially fixedly but rotatably to the sleeve 16 and theeccentric peg 19 by means of a screw 22 that is coaxial with theeccentric axis 19 and is screwed into the eccentric peg 18, and thegrinding plate can be released again by loosening the screw 22. Betweenthe tool housing 11 and the upper side, in terms of FIG. 1, of thegrinding plate 21, there is a cuff 23 that seals off the intermediatespace. The manual grinder 10 is equipped with an internal dustextractor, which includes a fan wheel 24 that is retained on the sleeve16 in a manner fixed against relative rotation. The fan wheel 24 islocated in a chamber 25, to which a dust extraction conduit 26 isconnected. The grinding plate 21 is provided with conduits and/oropenings 27 for the internal dust extraction.

[0036] The manual grinder 10 has an annular first rolling face 28,extending with the grinding plate unit 15 all the way around theeccentric axis 19; this face is embodied here as an outer annular gear29. This outer annular gear 29 can, in an exemplary embodiment notshown, be an independent component, such as a gear wheel, that isconnected to the sleeve 16 in a manner fixed against relative rotation.In the exemplary embodiment shown, the annular gear 29 is especiallysimply a part and in particular an integral part of the grinding plateunit 15, and in particular of the grinding plate 21, which in thisintegral design is advantageously formed of a plastic injection-moldedpart. Associated with the first rolling face 28 and in particular theouter annular gear 29 is an annular second rolling face 30, whichencircles the first rolling face 28 and whose center axis extendscoaxially to the central axis 17 of the work spindle 13. The firstrolling face 28 can roll along the second rolling face 30 when the drivemotor 12 has been switched on. The second rolling face 30 isadvantageously embodied as an inner annular gear 31 of an external crowngear 32. The second rolling face 30, and in particular the externalcrown gear 32 that carries it, is rotatably supported about the centralaxis 17. In the first exemplary embodiment shown, the external crowngear 32 has a bearing ring 33 of smaller diameter, disposed along thecentral axis 17 and axially spaced apart from the inner annular gear 31;by way of this bearing ring, the external crown gear 32 is supported onthe sleeve 16, rotatably relative to it, by means of a bearing 34, suchas a ball bearing. The inner ring of the bearing 34 is connected to thesleeve 16 in a manner fixed against relative rotation, while the outerring of the bearing 34 is connected to the bearing ring 33 and thus tothe external crown gear 32 in a manner fixed against relative rotation.

[0037] A braking device identified overall by reference numeral 35 isassociated with the second rolling face 30, and with it a rotation ofthe second rolling face 30 about its center axis, that is, about thecentral axis 17, can be selectively suppressed or enabled. Thearrangement is such that the rotatability of the second rolling face 30,in particular of the external crown gear 32, can be suppressed orenabled during tool operation by means of the braking device 35.

[0038] The external crown gear 32 is designed as an integral componentand advantageously comprises a lightweight metal die-cast part. Thesleeve 16 with the eccentric peg 18 on its end is advantageously madefrom a sintered part.

[0039] In the first exemplary embodiment shown, the fan wheel 24 isdisposed, as part of the internal dust extractor, in a manner fixedagainst relative rotation on the sleeve 16. In another exemplaryembodiment, not shown, the fan wheel 24 is instead embodied on a fansleeve, which is disposed on the work spindle 13 in a manner fixedagainst relative rotation and has an inner sleeve, which is eccentric tothe central axis 17 and in which, by means of a bearing coaxial with theeccentric axis 19, a cylindrical sleeve similar to the sleeve 16 isretained rotatably and axially fixedly; the grinding plate 21 isreleasably secured to its end by means of the screw 22.

[0040] The braking device 35 has a brake member 36, acting in brakingfashion on the second rolling face 30, and an actuating device 37 foractuating the brake member 36. The brake member 36 is provided with abraking face 38, with which the brake member 36 can superficially engagean associated face 39 of the second rolling face 30 in order to blockthe second rolling face 30 from rotating; this face 39 is embodied inparticular as an outer face of the external crown gear 32. The secondrolling face 30 extends on the outside and encircles the first rollingface 28; both of them extend essentially within a common planediametrically opposed to the central axis 17 of the work spindle 13. Theface 39, designed as an outer face, of the second rolling face 30 inparticular comprises the outer circumferential face 40 of the externalcrown gear 32.

[0041] The brake member 36 quite generally comprises a brake part of thekind that for blocking rotation can rest with its inside, embodied as abraking face 38, on the associated face 39 of the second rolling face30, and in particular on the outer circumferential face 40 of theexternal crown gear 32. In an especially simple design, the brake member36 comprises a brake band, which wraps around the outer circumferentialface 40 of the wk 32 and which can be tensed against the outercircumferential face 40 in order to block rotation.

[0042] The actuating device 37 has an eccentric lever, which has acentral part 41, supported pivotably in the tool housing 11, and aneccentric peg 42 thereon, which engages an opening 43, such as a loop,on one end of the brake member 36 designed as a brake band. The centralpart 41 is engaged by a handle 44, located outside the tool housing 11,for the sake of pivoting actuation. This actuating device 37, in theform of an eccentric lever as explained, is designed such that tensingof the brake member 36, embodied as a brake band, is effected by meansof pivoting of the central part 41 about a circumferential angle ofapproximately 180°. The tensed position, and thus the state of theexternal crown gear 32 in which it is blocked against rotation, is shownin FIG. 3. If the handle 44 is pivoted counterclockwise about 180° intothe position shown in FIG. 2, then the brake member 36 in the form ofthe brake band is relaxed, so that the external crown gear 32 is notblocked, and its rotation about the central axis 17, whichsimultaneously represents its center axis, is therefore made possible.

[0043] In another exemplary embodiment, not shown, the actuating device37 has a magnet, such as a controllable electromagnet, that actuates thebrake member 36, for instance in the form of a brake band, and inparticular that tenses the brake band.

[0044] Another special feature of the manual grinder 10 is that theinner annular gear 31 of the external crown gear 32 has a greater numberof teeth than the outer annular gear 29. The difference in the number ofteeth can for instance be two. This means that when the external crowngear 32 is braked, the outer annular gear 29 rolling along its innerannular gear 31 rotates onward by one tooth per 180° of eccentricmotion, and thus with respect to the grinding plate 21 provided with theouter annular gear 29, a rotary speed of the grinding plate arises inthe rolling process. For an assumed number of oscillations of 10,000 andwith a ratio of the number of teeth of 50:48, the rotary speed of thegrinding plate 21 is for instance 417 rpm.

[0045] If the braking device 35 is put in the braking position shown inFIG. 3 and if the drive motor 12 is turned on, then via this motor thedrive spindle 13 and the sleeve 16, together with the fan wheel 24 andany balancing masses that may be present, not particularly shown here,are driven to rotate, for example at a speed of approximately 10,000rpm. In the exemplary embodiment shown, the drive of the work spindle 13is effected directly onto the sleeve 16. In another exemplaryembodiment, not shown, a gear is instead connected between them. Becauseof this driving motion, the grinding plate 21 is driven in such a waythat in addition to the eccentric motion, a rotation about the eccentricaxis 19 takes place, the result of which is an eccentric rotary motionof the grinding plate 21. Because of the active braking device 35, theexternal crown gear 32 is prevented from rotating, so that uponrevolving, the outer annular gear 29 can roll along the inner annulargear 31. In this stage, a forced-drive mode of the grinding plate 21about the eccentric axis 19 as well is thus brought about.

[0046] If during the operation of the manual grinder 10, the brakemember 36 is now shifted to the non-braking state shown in FIG. 2 bymeans of the actuating device 37, then the external crown gear 32 isfreely rotatable about the central axis 17, because of the support onthe sleeve 16 by means of the bearing 34. The external crown gear 32 cannow rotate as well, because of the friction in the bearing 34. Dependingon the friction conditions, a relative motion of the external crown gear32 occurs in the opposite direction of rotation from the grinding plate21. The rotary speed of the grinding plate 21 is dependent on the loadon the underlying support, that is, on how solidly the manual grinder 10is pressed with the grinding plate 21, and a grinding blade releasablysecured to it, for instance by means of a Velcro fastener, against aworkpiece to be machined. Depending on given conditions, the rotaryspeed of the grinding plate 21 can also become zero. In that stage, thefree-wheeling mode results for the manual grinder 10.

[0047] During the operation of the manual grinder 10 while it isswitched on, a switchover from this free-wheeling mode to theforced-drive mode again can be made by actuation of the braking device35.

[0048] The manual grinder 10 described is simple, compact, andeconomical. In a simple way, by lever actuation, or in another exemplaryembodiment, not shown, by actuation of a magnet, a switchover to acontinuous transition from the forced-entrainment mode to thefree-wheeling mode is possible during tool operation. Because the outerannular gear 29 is injection-molded onto an annular part of the grindingplate 21 and is thus integral with it, the advantage is obtained of areduced number of components and a lesser structural height. The littleeffort and expense needed to achieve the rolling gear, comprising theouter annular gear 29 and the inner annular gear 31, is alsoadvantageous.

[0049] In FIG. 5, a second exemplary embodiment of a motor-driven manualgrinder 10 is schematically shown. The drive of the grinding plate unit15 and this grinding plate unit itself are in principle constructedidentically to those of the first exemplary embodiment. A work spindle13 is driven to rotate about a central axis 17 by a drive motor 12, notshown. On its power takeoff end, the work spindle 13 has an eccentricpeg 18. This peg forms an eccentric axis 19. A first rolling face 28 isembodied concentrically about the eccentric axis 19, and with its outerannular gear 29 it engages an inner annular gear 31 of a second rollingface 30, which is arranged concentrically around the central axis 17.The grinding plate unit 15 is connected via a coaxial screw 22 to apower takeoff shaft 4, which is disposed coaxially about the eccentricaxis 19. The parts described function like those of the first exemplaryembodiment, so that reference is made to their description there.

[0050] Unlike the first exemplary embodiment, the outer circumferentialface 40 of the second rolling face 30 is not embodied as essentiallysmooth but instead has a crown gear 8. The brake member 36 is embodiedas an elastic element 7 and, in a distinction from the substantiallysmooth brake member 36 of the first exemplary embodiment, is providedwith teeth 2, which are located opposite the crown gear 8.

[0051] In FIG. 6, the three-dimensional disposition between the teeth 2,which are embodied on the side of the elastic element 7 opposite thecrown gear 8, is clearly shown. Here the elastic element 7 is in itssecond position, in which its teeth 2 are not meshing with the crowngear 8 of the second rolling face 30. As a result, the second rollingface 30 can run freely in this decoupled state. This means that thegrinding plate 21 executes only an oscillating motion and a slightrotary motion, dependent on the bearing friction in the bearing 20. Themanual grinder 10 is accordingly in the fine grinding mode. The greaterthe bearing friction, the more pronounced is the rotary motion. On oneend, the elastic element 7 has a fixation point 3, which is connected tothe tool housing 11 via a fixed rotary axis 5. On its other end, theelastic element 7 is embodied as a detent lever 6. The detent lever 6 isactuated by means of an eccentric bolt 1, which is connected to anactuating device 37 (not shown) of the kind in the first exemplaryembodiment. The detent lever 6 is pressed constantly against theeccentric bolt 1 by a spring element 9, which is braced on the toolhousing 11. The teeth 2 of the elastic element 7 are embodied closer tothe detent lever 6 than to the fixation point 3. The fixation point 3and the detent lever 6 are located essentially diametrically oppositethe central axis 17. For the angle a between the fixation point 3 andthe teeth 2, this means that the angle is greater than 90°. Not onlydoes this prevent overlooking of the teeth 2 opposite the crown gear 8under load, but it also enhances a retention force between the teeth 2and the crown gear 8, in the manner of a servo effect.

[0052] In FIG. 7, the elastic element 7 is shown in its first position.The teeth 2 of the elastic element 7 engage the crown gear 8 on theouter circumferential face 40 of the second rolling face 30 by positiveengagement. A forced entrainment is thus achieved, and the manualgrinder 10 is operating in the coarse grinding mode. In that case, arotary motion is imposed on the oscillating motion of the grinding plate21. Because of the positive engagement between the teeth 2 and the crowngear 8, there is no slip between the braking device 35 and the secondrolling face 30, and thus both wear and heat production tend towardzero. The engagement between the teeth 2 and the crown gear 8 is broughtabout, during the transition from the second position (FIG. 6) to thefirst position (FIG. 7), by providing that the eccentric bolt 1 isshifted, by the actuating device 37, not shown, from its position shownin FIG. 6 into its position shown in FIG. 7. For the motion of theeccentric bolt 1 from its position shown in FIG. 7 into its positionshown in FIG. 6, the description made in conjunction with the firstexemplary embodiment applies here as well. In the process, the springelement 9 presses the detent lever 6 constantly against the eccentricbolt 1. Since the one end of the elastic element 7 is supportedrotatably about the fixed rotary axis 5, the elastic element 7 isbrought closer to the second rolling face 30 before the positiveengagement comes about. In the process, the eccentric bolt 1 is movedapproximately 4° relative to the central axis 17.

[0053] The user thus has the capability of changing from the finegrinding mode to the coarse grinding mode via a switch lever, withouthaving to turn off the tool. The switchover from free-wheeling to forcedentrainment can be made, by means of an elastic intermediate coupling asdescribed, in all operating states, that is, idling, at a stop, andunder load. Moreover, the switchover is simple and involves littletolerance.

[0054] In FIG. 8, the elastic element 7 is embodied in a version that isespecially simple to realize. It is shown in its first position. Theelastic element 7 is embodied as a shaped part of spring steel, which isessentially in the shape of a circular arc. On its first end 51, it isin a torsion spring joint 50, which is embodied rigidly with the toolhousing (not shown). The elastic element 7 has a first recessed region45, which is embodied as a set of teeth 46. In the exemplary embodimentshown, there are two teeth. However, only a single tooth and more thantwo teeth, for instance three or four teeth, are equally good options.The elastic element 7 is fastened in the torsion spring joint 50 in sucha way that by its spring force, with its set of teeth 46, it pressesagainst the crown gear 8 of the outer circumferential face 40 of thesecond rolling face 30. The crown gear 8 as a result snaps into theteeth 46 of the elastic element 7. This positive-engagement connectioncan be undone by exerting a force 49 on the second end 48 of the elasticelement 7, which force undoes the locking by moving the teeth 46 awayfrom the crown gear 8. This is accomplished counter to the spring forceof the elastic element 7, because of its fastening in the torsion springjoint 50. Besides the first recessed region 45, the elastic element 7has a second recessed region 47. This second recessed region 47 has atangential elasticity and thus serves as a rotary impact cushion.

[0055] Such a design of the braking device 35 is very simple andeconomical. It can be actuated very simply, with the expenditure of onlyslight actuating force, and furthermore has a rotary impact cushion andan overload protection for the forced operating mode. Besides theadvantage of requiring little space, it allows compensating for errorsin pitch of the gear part, caused for instance by wear.

LIST OF REFERENCE NUMERALS

[0056] 1 Eccentric bolt 2 Teeth 3 Fixation point 4 Power takeoff shaft 5Fixed rotary axis 6 Detent lever 7 Elastic element 8 crown gear 9 Springelement 10 Manual grinder 11 Tool housing 12 Drive motor 13 Drivespindle 14 Bearing 15 Grinding plate unit 16 Sleeve 17 Central axis 18Eccentric peg 19 Eccentric axis 20 Bearing 21 Grinding plate 22 Coaxialscrew 23 Cuff 24 Fan wheel 25 Chamber 26 Dust extraction conduit 27opening 28 First rolling face 29 Outer annular gear 30 Second rollingface 31 Inner annular gear 32 External crown gear 33 Bearing ring 34Bearing 35 Braking device 36 Brake member 37 Actuating device 38 Brakingface 39 Face 40 Outer circumferential face 41 Central part 42 Eccentricpeg 43 Opening 44 Handle 45 First recessed region 46 Set of teeth 47Second recessed region 48 Second end 49 Force 50 Torsion spring joint 51First end

1. A motor-driven manual grinder, in particular an eccentric-plategrinder, having a work spindle (13) supported in the tool housing (11),and having a grinding plate unit (15) driven and rotatable by the workspindle to execute an eccentric motion, and having an annular firstrolling face (28) on the one hand, extending together with the grindingplate unit (15) all the way around the eccentric axis (19), and anannular second rolling face (30) on the other, associated with thefirst, whose center axis extends coaxially to the central axis (17) ofthe work spindle (13) and on which the first rolling face (28) can roll,characterized in that the second rolling face (30) is supported,rotatable about its center axis, in a bearing (34), and that a brakingdevice (35) is associated with the second rolling face (30), by means ofwhich device a rotation of the second rolling face (30) about its centeraxis can be selectively prevented or enabled.
 2. The motor-driven manualgrinder of claim 1, characterized in that the rotatability of the secondrolling face (30) can be prevented or enabled by the braking device (35)during tool operation.
 3. The motor-driven manual grinder of one of theforegoing claims, characterized in that the braking device (35) has abrake member (36), acting in braking fashion on the second rolling face(30), and an actuating device (37) for actuating the brake member (36).4. The motor-driven manual grinder of one of the foregoing claims,characterized in that the brake member (36) has a braking face (38),with which the brake member (36) can superficially engage an associatedface (39) of the second rolling face (30), especially an outer face, inorder to block the second rolling face (30).
 5. The motor-driven manualgrinder of one of the foregoing claims, characterized in that the secondrolling face (30) extends on the outside and encircles the first rollingface (28), and that both rolling faces (28, 30) extend essentiallyinside a common plane diametrically opposed to the axis (17) of the workspindle (13).
 6. The motor-driven manual grinder of one of the foregoingclaims, characterized in that the second rolling face (30) is embodiedas an inner annular gear (31) of an external crown gear (32), and thefirst rolling face (28) is embodied as an outer annular gear (29). 7.The motor-driven manual grinder of one of the foregoing claims,characterized in that an outer face, in particular an outercircumferential face (40) of the second rolling face (30), in particularof the external crown gear (32), is embodied as a face that cooperateswith the brake member (36) to block rotation.
 8. The motor-driven manualgrinder of one of the foregoing claims, characterized in that theactuating device (37) has a magnet, in particular a controllableelectromagnet, that actuates the brake member (36), or has an eccentriclever, which for instance is manually actuatable, with an eccentricelement (42) acting on the brake member (36).
 9. The motor-driven manualgrinder of one of the foregoing claims, characterized in that the brakemember (36) of the braking device (35) can rest with an inside face,embodied as a braking face (38), on the associated face (39), inparticular on the outer circumferential face (40) of the external crowngear (32), to block rotation.
 10. The motor-driven manual grinder of oneof the foregoing claims, characterized in that the brake member (36) isformed of a brake band, which for blocking rotation can be tensedagainst the face (39), in particular the outer circumferential face (40)of the external crown gear (32).
 11. The motor-driven manual grinder ofone of the foregoing claims, characterized in that the brake member (36)can be locked relative to the tool housing (11) in a first position, inwhich it is in positive engagement with the second rolling face (30),and in a second position, in which it is not in any engagement with thesecond rolling face (30).
 12. The motor-driven manual grinder of claim11, characterized in that the brake member (36) is a band, in particulara toothed belt, with a plurality of teeth (2) which are able to enterinto engagement with a crown gear (8) of the second rolling face (30).13. The motor-driven manual grinder of claim 11 or 12, characterized inthat the brake member (36) is embodied as an elastic element (7), inparticular as a toothed belt.
 14. The motor-driven manual grinder of oneof claims 11-13, characterized in that the brake member (36), inparticular the elastic element (7), is connected to the tool housing(11) at a fixation point (8) such that it is rotatable about a fixedrotary axis (5).
 15. The motor-driven manual grinder of one of claims11-14, characterized in that the braking device (35) has a detent lever(6), which is connected to the tool housing (11) via a spring element(9).
 16. The motor-driven manual grinder of claim 15, characterized inthat the spring element (9) seeks to press the detent lever (6) into aposition in which the elastic element (7) assumes its first position.17. The motor-driven manual grinder of one of claims 11-16,characterized in that the brake member (36) is of spring-elasticmaterial and has a first recessed region (45) with a set of teeth (46)which can be brought into engagement with the outer circumferential face(40), and a second recessed region (47), and the brake member (36) isheld by prestressing in its first position and can be brought into itssecond position by an actuating device (37).
 18. The motor-driven manualgrinder of one of claims 11-17, characterized in that the detent lever(6) is displaceable by means of an eccentric bolt (1), which issupported rotatably on the tool housing (11) and is operable by means ofthe actuating device (37) protruding from the tool housing (11).
 19. Themotor-driven manual grinder of one of claims 14-18, characterized inthat the fixation point (3) and the eccentric bolt (1) are essentiallydiametrically opposite one another relative to the central axis (17),and the angle (a) between the fixation point (3) and the teeth (2) ofthe elastic element (7), which in the first position are in engagementwith the second rolling face (39), is greater than 90°.
 20. Themotor-driven manual grinder of one of claims 14-19, characterized inthat the fixation point (3) and the eccentric bolt (1) are essentiallydiametrically opposite one another relative to the central axis (17),and the cooperating faces of the brake member (36) and of the secondrolling face (30), under load, reinforce the retention force, in amanner similar to a servo effect.
 21. The motor-driven manual grinder ofone of claims 11-20, characterized in that the braking device (35) isswitchable between the first and second position and vice versa in alloperating states, and in particular during idling, at a stop, and underload.